The present invention relates in general to microcatheters and more specifically to catheters intended for administering spinal anesthesia which possess markedly improved tensile strength.
Microcatheters are catheters having an external diameter less than 0.022 inches or 0.559 mm and an internal diameter greater than 0.004 inches or 0.102 mm. Such catheters are predominantly used in the vertebrae region to administer continuous spinal, epidural and caudal anesthesia. The anatomical and physiological nuances encountered in the vertebrae column in general mandate a flexible, small diameter catheter on the one hand and on the other hand, one with structural integrity, particularly tensile strength to enable threading through a spinal needle and to prevent catheter breakage upon removal of the catheter from the spinal column.
As an example of such a microcatheter, mention may be made of the "CoSPAN" (trademark of The Kendall Company, assignee of the instant invention) for a 28 gauge nylon catheter which can be inserted with a 22 gauge needle. The catheter can dispense 1 ml of fluid in less than a minute as well as being employed to aspirate fluid. It possesses a mean breaking force of on the order of about 1.8 pounds force.
As used herein, the term "breaking force" is the maximum force in pounds which the specimen can bear before breaking. Tensile strength, then, is the breaking force divided by the cross-sectional area, which may then be represented by the formula: ##EQU1## where TS=tensile strength; BF=breaking force; and X=cross-sectional area
As a further example of commercially available microcatheters for spinal anesthesia, mention may be made of the 28 gauge polyurethane microcatheter made by Preferred Medical which possesses a tensile strength comparable to that of the aforementioned CoSPAN microcatheter.
While not intended to represent an exhaustive survey of the patent literature, the following patents representing a cursory search may nevertheless be taken as illustrative.
U.S. Pat. No. 3,780,733 teaches (1) first partially inserting a 15 gauge needle into the dural space; (2) guiding a 25 gauge needle coupled to a 20 gauge catheter through the 15 gauge needle lumen into the subarachnoid space with the use of stylet inserted in the catheter.
While the actual puncture created in the subarachnoid space with the 25 gauge needle is small, disadvantages exist. For one, the needle may discouple from the, catheter and become dislodged in the spinal column. Secondly, the technique disadvantageously requires that the metal needle be maintained in the subarachnoid space throughout the surgery thereby creating possible injuries due to physical movement of the metal needle. Thirdly, the metal guiding stylet may cause impact trauma during insertion.
U.S. Pat. No. 4,917,670 teaches a polymeric microcatheter having an external diameter less than about 0.0120 inch (0.305 mm), so that it will fit through a thin wall 24 to 26 gauge spinal needle, with a reinforcing stylet inserted or affixed therein. While this invention alleviates the discoupling risk and prolonged insertion of the metal needle for the duration of the surgery, it still requires the use of a metal stylet.
Lastly U.S. Pat. No. 3,634,924 claims, "[T]he method of making a balloon catheter comprising providing a catheter tube of a thermoplastic material having a memory characteristic and containing a balloon inflation lumen, applying heat to an end portion of said tube sufficient to soften said material, drawing out and elongating said softened portion to shrink its diameter, applying a pair of rigid ferrules over said shrunken portion, applying heat to said shrunken portion to reexpand its diameter and lock said ferrules on said tube, forming a balloon inflation opening in said material communicating with said lumen, applying a sleeve of elastic balloon material over said reexpanded portion, and binding said balloon material to said ferrules."
Spinal anesthesia is a commonly employed anesthesia for almost any type of major procedure below the level of the diaphragm. Spinal anesthesia involves blockage of the nerve roots and spinal cord to prevent transmission of nerve impulses. Saddle block anesthesia or low spinal anesthesia is similar to spinal anesthesia except that the anesthetic agent is injected between the 3rd and 4th lumbar space as opposed to the 2nd and 3rd lumbar space in spinal anesthesia and, therefore, mainly produces anesthesia in the perineal area. Caudal block anesthesia in contract produces more extensive anesthesia than saddle block, namely from the umbilicus to the toes.
Spinal anesthesia involves a lumbar puncture and spinal tap. The lumbar puncture is performed by an anesthesiologist between the second and third lumbar vertebrae.
The procedure is begun by prepping the patient's skin surrounding the puncture site with an antiseptic and draping the patient with a sterile towel. Next a local anesthetic is administered to the site to produce a local nerve block. Once the area is anesthetized, a spinal needle is inserted and a local anesthetic agent is injected through the spinal needle into the subarachnoid space between the pia mater and the arachnoid membrane into the spinal fluid.
If the procedure is predicted to take longer than 3-4 hours, which is the time of the longest acting single dose anesthetic agent, a continuous spinal anesthesia is preferred to avoid repeated lumbar punctures. A continuous spinal anesthesia involves the insertion of a semi-rigid catheter threaded through the spinal needle and into the subarachnoid space. This procedure advantageously allows for intermittent anesthetic administration since the catheter remains in place throughout the operation.
In general, the microcatheters of the prior art tend to possess an undesirably low tensile strength. While the tensile strength may be increased to a point by selection of the polymeric material employed in its synthesis, e.g. the nylon from which the CoSPAN is made, there is still a need in the art to increase the tensile strength of these thin walled catheters still further. Accordingly, the microcatheters of the prior art either utilize a reinforcing metal wire embedded in the polymeric catheter wall to enhance tensile strength or, if wireless, the tensile strength is sufficiently low to present the inherent danger of breakage on removal. The consequences of having catheter parts left in the spinal column are obviously disastrous.
As heretofore mentioned, the nylon material utilized in the manufacture of the CoSPAN spinal catheter will provide a 28 gauge catheter having mean breaking force slightly in excess of 1.5 pounds. While this tensile strength is entirely satisfactory in most spinal anesthesia procedures, breakage can and will still occur, mainly due to human error.
For example, breakage is often the result of an inadvertent attempt to withdraw the catheter backward against the needle bevel. Strict avoidance of such back-pulling should minimize the rare incidence of this complication. Another cause of breakage is the compressive forces which retain the catheter (which is referred to as the "Laminer Pincer"). This cause of breakage can be minimized by flexing the patient's spine while the catheter is being gently removed. Knotting of the catheter, which may also contribute to catheter breakage, can be minimized by properly inserting no more than 2 to 4 cm of catheter into the subarachnoid space.
In those infrequent instances where breakage of the CoSPAN catheter does occur, the patient must first be informed and reassured. X-rays may be used to locate the segment of broken catheter, and if it is just below the skin line, it can be removed under local anesthesia.
On the other hand, while the use of a reinforcing wire embedded in the catheter wall may increase the tensile strength to a level preventing breakage on removal, the presence of this reinforcing wire also presents inherent danger to the patient. For example, if the catheter having a reinforcing wire is bent on insertion, it stays bent, thereby inhibiting the flexibility needed for insertion. If it is kinked, it will stay kinked, thus lowering the flow rate for the spinal anesthesia or other liquid to be administered. Further, it the wire causes the catheter to be relatively stiff, it may track straight ahead on insertion and possibly embed itself in the tissue wall of the dura mater or, even worse, puncture it to cause a leak of spinal fluid, nerve damage, or both.
Accordingly, the task of this invention, simply stated, is to provide microcatheters for medical procedures such as continuous spinal anesthesia which are characterized by a substantially greater breaking force sufficient to materially inhibit if not preclude breakage on removal, which microcatheters are accordingly wireless and thereby obviate the dangers inherent in the use of reinforcing wires.